The invention relates to an optical switching device comprising:
a substrate, PA1 a switching film formed on the substrate, which switching film comprises a hydride of a trivalent transition or rare earth metal and can be reversibly switched from a light-absorbing dihydride state to a transparent trihydride state by an exchange of hydrogen, PA1 a capping layer formed on the switching film, the capping layer being capable of hydrogen dissociation, and PA1 a hydrogen source which is in contact with the capping layer.
The invention also relates to the use of such a switching device.
In optical switching devices, the optical properties are governed by external influences, such as gas pressure, an electric voltage or current, temperature or light.
For example, electrochromic devices are well-known in which a layer of an electrochromic material, such as MoO.sub.3, is sandwiched between two transparent electroconductive electrode layers, for example, of indium-tin oxide. A layer of an H.sup.+ - or Li.sup.+ -ion-conducting material is present between an electrode layer and the electrochromic material. The device also comprises an ion-storage layer for storing ions. The application of an electric current perpendicular to the layers causes the transmission or colour of the layer packet to change. Said transmission change is reversible. Electrochromic materials are used, for example, in variable-transmission windows for buildings and anti-dazzle mirrors in cars.
A drawback of oxidic electrochromic devices is that an extensive stack of layers is required for their operation. A further important disadvantage is that such materials enable only a small transmission change, and hence a small contrast, to be attained.
U.S. Pat. No. 5,635,72 filed by applicants, a switching device is disclosed in which a switching film of a trivalent transition or rare earth metal, such as Y and La, coated with a palladium capping layer, can be reversibly switched between a low hydrogen state (YH.sub.x ; x.apprxeq.1.7-2.5) and a high hydrogen content (x.apprxeq.2.5-3) by an exchange of hydrogen. The low hydrogen state is metallic and light absorbing, whereas the high hydrogen state is transparent. Switching of the film to the transparent state is carried out by varying the partial pressure of hydrogen gas (e.g. between 1 bar and vacuum) in contact with the Pd capping layer, which layer dissociates H.sub.2 -molecules into H-atoms. At a hydrogen pressure of 1 bar, the absorbent low hydrogen state is converted to the transparent high hydrogen state. After evacuation of hydrogen, the high hydrogen state is converted to the low hydrogen state. These conversions take place within a few seconds. In the following part of this document, the designations dihydride and trihydride state will be used, which correspond to a low hydrogen and high hydrogen state, respectively.